With the deployment over the years, it may be considered that 2G/3G networks provide full coverage to some extent. As System Architecture Evolution (SAE)/Long Term Evolution (LTE), High Speed Packet Access (HSPA), HSPA+, and Worldwide Interoperability for Microwave Access (WiMAX) network technologies develop, these networks have already cover some urban areas and traffic hotspots. Consequently, in a current communications network, these networks coexist with the 2G/3G networks. For ease of description, SAE/LTE, HSPA, HSPA+, WiMAX, and the like that provide only a packet switched (PS) domain are collectively referred to as PS domain-only networks. For different network bearer mechanisms, voice calls of mobile communication may be classified into the following several types:
A first type is a conversational service borne by a circuit switched (CS) domain, such as a voice call.
A second type is a conversational service home by a PS domain, which is also referred to as a Voice over Internet Protocol (Internet Protocol, IP) (Voice over Internet Protocol, VoIP) service, and quality of service (Quality of Service, QoS) is ensured by an Internet Protocol (Internet Protocol, IP) multimedia subsystem (IP Multimedia Subsystem, IMS), such as a voice call over IP.
With regard to a PS domain-only network, a voice call can be transmitted on only a PS domain bearer of the PS domain-only network, and an IMS controls the voice call by using an IP-based network. On a 2G/3G network, a voice call is generally borne in a CS domain.
In an early stage of LTE deployment, an operator can provide a voice service for a user by using a 2G/3G network and provide a data service for a user by using an LTE network. A terminal is a multi-mode terminal; however, because of complex implementation and consideration of costs, most terminals cannot simultaneously send and receive an LTE signal and a 2G/3G signal, which is solved in the prior art by using a circuit switched fallback (CS fallback, CSFB) technology. In a scenario, a UE performs a data service or is in an idle state on an LTE network, and in this case, to initiate a CS voice call, the UE needs to switch a wireless module of the UE from LTE to 2G/3G, and then the UE initiates a voice call on the 2G/3G. This call is referred to as a call initiated by the UE (MOC, mobile originating call). In another scenario, a UE performs a data service or is in an idle state on an LTE network, and in this case, a user needs to call the UE, and the LTE network needs to be capable of notifying the UE that a CS call arrives, so that the UE can switch a wireless module to 2G/3G to receive the CS call. This call is referred to as a call in which the UE is a callee (MTC, mobile terminating call).
When combined registration is performed by using an interface SGs between an MME and an MSC server, it can be implemented that the UE is registered, by using the MME, with an MSC while being attached to the LTE. Subsequently, the UE can initiate a CS service after falling back to a CS domain. The prior art for the UE to fall back to the CS domain includes: packet switched (PS) domain switch, network assisted cell change (NACC), and radio resource control release (RRC release).
The prior art does not provide a technical solution in which a UE initiates a CS call or receives a CS call after falling back to a circuit switched domain by switching from a PS domain to the CS domain.